Circuit breakers and switches for interrupting or making an electrical circuit when excessive current is drawn, or when electrically-operated equipment becomes too warm are known, and typically include a laminated bimetallic element, the two materials of the laminated bimetallic element having different coefficients of expansion with temperature, so that the bimetallic element curls and straightens as its ambient temperature is raised and lowered, by variations in surrounding air temperature, variation in current flowing through the bimetallic element, or current flowing through an associated heating resistor. Conventionally, this bimetallic element carries the movable element of a contact pair, which is moved towards or away from a fixed contact as temperature changes, and gradually establishes or breaks contact with the fixed contact, to allow or stop the flow of current through the fixed and stationary contacts. As will be apparent, gradual engagement and disengagement causes rapid wear of the contacts and early failure of the circuit breaker. To overcome this problem, bimetallic elements have been formed with cutout sections, or with curved cross sections, to provide a snap action.
However, heating the bimetallic element affects the characteristics of the bimetallic element. The materials of the bimetallic element are heat-treated, mechanically-treated and tempered, for particular granular characteristics that give the metal its desired properties of hardness, resilience, and so forth. Heating causes undesirable but unavoidable further tempering, which causes a gradual growth in grain size, and a gradual change in temperature and fatigue-resistant characteristics of the material. Thus, the operating temperature of a conventional bimetallic switch may change over time, and the bimetallic element may fail prematurely due to decreased fatigue resistance.
It has also been proposed to use the Curie temperature characteristics of a magnetic material to provide a temperature switch, such as in soldering iron temperature switches disclosed in U.S. Pat. Nos. 2,951,927 and 3,287,541, to operate a separate switching mechanism. However, magnetic materials previously used for their Curie temperature characteristics do not switch from magnetic to paramagnetic or non-magnetic characteristics abruptly, but rather begin a change at a measured Curie temperature, and complete the change at a higher temperature, leading to the disadvantage of gradual switch operation. It is also known that the Curie temperature of a magnetic material may depend on its prior history, in terms of mechanical working, heat-treating and tempering. The instant invention provides a solution to these and other problems of known circuit breakers.